Fix the bitcode upgrade for DIGlobalVariable in a DIImportedEntity context.

The bitcode upgrade for DIGlobalVariable unconditionally wrapped
DIGlobalVariables in a DIGlobalVariableExpression. When a
DIGlobalVariable is referenced by a DIImportedEntity, however, this is
wrong. This patch fixes the bitcode upgrade by deferring the creation
of DIGlobalVariableExpressions until we know the context of the
DIGlobalVariable.

<rdar://problem/30134279>

Differential Revision: https://reviews.llvm.org/D29349

llvm-svn: 294318